The present invention relates to a method for operating an internal combustion engine, in particular a diesel internal combustion engine, and to an injection apparatus therefore.
When operating internal combustion engines with direct injection, attempts have been made in the past to influence the combustion and the formation of emissions by varying the injection profile. In the case of direct-injection internal combustion engines with compression ignition, injection valves in which the configuration of the injection profile is controlled by a suitable structure and targeted opening of the injection valve are used to configure a load-dependent fuel injection. This is also intended to improve and optimize the functioning of a downstream exhaust-gas aftertreatment. By way of example, a targeted injection profile shaping can be used to provide a substoichiometric exhaust gas for desulfurizing NOx storage catalytic converters and for the on-board generation of NH3.
DE10159479.8-13, a patent application which was not published before the priority date of the present patent, has disclosed a method in which fuel is introduced into a combustion chamber as a main injection and a postinjection, it being possible for the two injections to be carried out cyclically. In this case, the intention is to influence the time profile of the combustion, so that a time sequence of the torque profile and/or a pressure profile in the cylinder of the internal combustion engine are altered in order to influence the exhaust gas composition and an exhaust gas temperature.
DE 19953932 A1 has disclosed a method in which a proposed combined homogeneous/heterogeneous operating mode of an internal combustion engine is intended to achieve medium and relatively high powers. In this case, both an early homogeneous mix formation in the compression stroke and a subsequent heterogeneous mix formation around the top dead center are supposed to be possible using one injection strategy, with the fuel injection for the homogeneous mix formation taking place at a lower injection pressure than for the heterogeneous mix formation, in order to avoid fuel being applied to the cold combustion chamber walls. Nevertheless, it has been found that despite the measures proposed above, high exhaust emissions continue to occur. Consequently, further measures have to be taken to minimize the exhaust emissions.
An object of the present invention is to provide a method for an internal combustion engine with compression ignition which reduces the exhaust emissions. According to the invention, this is achieved by a cyclically injected postinjection in partial quantities such that the partial fuel quantities are formed in different magnitudes.
A further object of the invention is to provide an apparatus which improves the exhaust gas properties and consumption properties of a compression-ignition internal combustion engine. According to the invention, this object is achieved by an apparatus having a spray hole cone angle of from 80° to 140° set between the injected fuel jets.
According to the method of the invention, fuel is injected directly into a combustion chamber of an internal combustion engine in the form of a plurality of fuel jets by an injection nozzle which has a nozzle needle and injection bores. A fuel quantity is injected at a later time following the main injection as a cyclical postinjection, the postinjection being injected cyclically in partial quantities, so that the partial quantities of fuel of the postinjection are formed in different magnitudes. As a result, it is possible to adapt the partial quantity of fuel in a targeted way to the piston position in the cylinder and/or an operating point of the internal combustion engine, with the respective partial quantity being mixed with the combustion chamber air in good time before it reaches the cylinder wall. As a result wetting of the cylinder wall with fuel is substantially avoided.
According to one configuration of the invention, during the cyclical postinjection, a lift of the nozzle needle of the injection nozzle and/or a fuel injection pressure are set so that for each partial quantity of the postinjection injected into the combustion chamber the reach of the respective fuel jet in the combustion chamber is limited and thereby the reach is less than the distance to a combustion chamber boundary. As a result, fuel accumulation on the walls of the cylinder, which rises constantly for example with a lower gas pressure and lower temperature in the cylinder, is minimized.
According to the invention, increased breaking-up and vaporization of the injection jet is achieved. The cyclical postinjection provided according to the invention leads to a shortening of the liquid jet length. This represents the length or penetration depth of an injection jet formed from liquid fuel. Therefore, application of fuel to the cold combustion chamber walls is avoided, because this fuel fraction mostly reacts with the residual air and residual gas in the cylinder, and therefore does not enter the engine oil, as is the case with increased introduction of fuel.
According to yet a further configuration of the invention, a first partial quantity of fuel of the postinjection is made to be greater than a subsequent quantity of fuel of the postinjection. This counteracts locally intensive enriching of the mix formed in the combustion chamber, in particular during the cyclical postinjection, so that the formation of particulates is minimized or prevented in particular during the postinjection. It is expedient for the individual injection quantities during the postinjection to be configured such that in each case a certain quantity of fuel enters the combustion chamber and is intensively and completely mixed with the combustion chamber air before the fuel jet has reached the combustion chamber boundary or a cylinder wall.
According to still a further configuration of the invention, the postinjection is injected into the combustion chamber at a lower injection pressure than that of the main injection. It is in this way possible to avoid further application of fuel to the walls, since a combustion chamber back pressure changes to a decreasing extent with a change in piston position during the postinjection. Furthermore, the pressure of the injected fuel during the cyclical postinjection can be varied or continuously adapted, preferably as a function of the piston position, or reduced to a lower level than during the main injection, in order to counteract the falling combustion chamber pressure during an expansion stroke of the internal combustion engine. As a result, by way of example. The depth of penetration of the partial quantities of fuel in the form of injection jets in the combustion chamber can be kept constant during the cyclical postinjection.
According to a further configuration of the invention, a lift of the nozzle needle of the injection nozzle is set so that a non-steady-state, cavitational flow is generated in the injection bores of the injection nozzle. The drops of fuel within the injected jet shortly after the jet leaves the injection nozzle and to be atomized in good time. This feature substantially minimizes the extent to which the fuel strikes, for example, the cylinder wall serving as a combustion chamber boundary.
According to the present invention, the cyclical nature of the postinjection is such that the reach of the fuel jet in the combustion chamber is limited for each partial quantity injected. As a result, the reach is substantially limited approximately to less than the distance to a cylinder wall by boosting the extent to which the fuel jets injected break down in the combustion chamber. During the postinjection, the individual injection cycles are configured in such a manner that in each case the jet momenta of the individual injections are adapted, and at the respective combustion chamber gas density the reach of the fuel jets approximately amounts to the distance to the combustion chamber cylinder wall or the piston base.
Control of an injection jet momentum and of an injection partial quantity is preferably effected by the pulse duration or the cycle time in combination with a targeted configuration of the injection nozzle needle, so that the fuel jets, as a result of increased atomization, break down shortly after they have emerged from the injection nozzle. The formation of particulates and significant application of fuel to the cylinder wall are substantially avoided or minimized.
Further criteria for the configuration of an additional postinjection can ensue from the demands of any exhaust-gas aftertreatment measure.
To carry out the method according to the invention, an inwardly opening nozzle needle with a plurality of injection bores is provided, in which the fuel is injected into the combustion chamber in the form of fuel jets through the injection bores, so that a spray hole cone angle of from 80° to 140° or from 80° to 160° can be set between the injected fuel jets.
According to one configuration of the method according to the invention, the lift of the nozzle needle in an opening direction is adjustable. Thus, during the cyclical postinjection the lift of the nozzle needle can be set variably. The setting of the lift can optionally take place in a load-dependent manner. As a result, an injection partial quantity which has been implemented during the cyclical postinjection is varied. Furthermore, a non-steady-state cavitational flow can be formed in the injection bores of the injection nozzle by the adjustment of the lift.